József Miró

Role of oxygen vacancy defect states in the n-type conduction of β-Ga2O3

Based on semiempirical quantum-chemical calculations, the electronic band structure of β-Ga2O3 is presented and the formation and properties of oxygen vacancies are analyzed. The equilibrium geometries and formation energies of neutral and doubly ionized vacancies were calculated. Using the calculated donor level positions of the vacancies, the high temperature n-type conduction is explained. The vacancy concentration is obtained by fitting to the experimental resistivity and electron mobility.

Theoretical and isotopic infrared absorption investigations of nitrogen-oxygen defects in silicon

The vibrational spectroscopy of NNO defects in Si introduced by 16O, 14N and 15N ion implantation is studied, and especially the N-isotopic shifts of the localized vibrational modes. These investigations show that the local modes of the three impurity atoms comprising the defect are only weakly coupled dynamically. Ab initio cluster calculations of the local mode frequencies of the defect are performed. Several models are investigated, and the model consisting of a bridging O atom adjacent to the N pair defect accounts for its dynamic properties.

The nitrogen-pair oxygen defect in silicon

The nitrogen-pair oxygen defect in silicon has been studied by infrared absorption spectroscopy on samples implanted with various combinations of 14N, 15N, 160 and 170. The measurements give direct evidence for the involvement of nitrogen and oxygen in the defect and show that the impurity atoms comprising the defect are only weakly coupled. Ab initio cluster calculation on several models of the nitrogen-pair oxygen defect have been performed and are compared with experiment. Based on these investigations a model consisting of a bridging oxygen atom adjacent to the nitrogen pair is suggested.

Theoretical studies on nitrogen - oxygen complexes in silicon

Semi-empirical PM3 cluster calculations are used to show that stable, electrically active NO complexes may exist in silicon. Based on their relative stability with respect to oxygen and nitrogen pairs, the retardation of thermal double donor formation in the presence of nitrogen is explained, but an equilibrium concentration much less than that of NN pairs is predicted. It is also shown that interaction of NO with a single nitrogen atom creates a bistable NNO defect, while encounter with an oxygen or an NN pair preserves the electrical activity of the NO centre. The possible role of the NO complex in shallow thermal donor formation is discussed.

Cyclic cluster model for calculating defects in solids using the local density approximation

The cyclic cluster model (CCM) was introduced earlier for defect calculations in the framework of semi-empirical quantum chemical methods with only two-centre interactions. The CCM represents a good balance between an approximate description of localized and extended states, offering most of the advantages of supercell methods at the cost of a molecular cluster calculation. This paper presents the adaptation of the CCM to an ab initio density functional Hamiltonian, incorporating the local density approximation, norm-conserving pseudopotentials and a Gaussian basis. Test results on perfect diamond cyclic clusters illustrate the advantages of the CCM.

Recombination with larger than bandgap energy at centres on the surface of silicon microstructures

Previously it has been shown that the correlation between shifts in luminescence and Raman peak positions in various porous silicon samples cannot be explained by the size effect of the silicon microstructures, as postulated by the (physical) quantum confinement model. It is also unlikely that siloxene occurs in significant quantities in porous Si. In this contribution, a possibility is presented for radiative recombination of carriers on the surface of a silicon microstructure, to explain strong visible luminescence in porous silicon. Nanometre-size structures on the surfaces of silicon microcrystallites introduce localized resonances into the (bulk-like) bands of the crystallites, giving rise to larger-than-bandgap transitions. These surface structures act as electron- and hole-traps and, due to localization of the carriers, recombination is possible neglecting momentum selection rules. Results of semi-empirical calculations on model structures are presented.

Shallow N-O Donors in Silicon

Shallow thermal donors (STD) are electrically active, oxygen-related, thermally formed defects observed in silicon after annealing between 300 and 600 °C. There are experimental indications that STD’s might be related to nitrogen. EPR studies find C2v symmetry for such defects. Recently, theoretical investigation on an NO complex has shown that it is a shallow donor. We discuss the possibility of adding an oxygen interstitial to the NO complex. By using semi-empirical cyclic cluster model calculations, two metastable structures have been found. One of these has C2v symmetry. The approximate spin distributions are reported for both of them.